Seam Welding

Questions and Answers

A stitch weld is a series of spot welds made in a row down two pieces of material. It is similar to a seam weld but is a weld gun or machine cylinder applying the individual welds rather than a rolling seam welding wheel.

A1 174b Roll Spot Weld 

VARY SPOT SPACING TO MAKE A STITCH WELD

These welds can be a lap joint or to join metal together as shown above. How long will the weld take?  This is determined by the weld schedule necessary for one weld. From this one schedule one would multiple by the number of welds necessary and add the move time between welds.

Using the figure above count the number of desired welds, times the weld time, and add in the time to move/fixture to the next position.

The upper figure has many more welds. It will obviously take longer than the figure on the lower part since there are fewer welds. Indexing the position in either circumstance can be automated and done quickly. That is the time to weld the above part.

I have seen a furnace blower cage end piece welded without weld placement precision in 1-3 seconds. The operator fired the welder like a machine gun for a second or so and flipped to the other end did it again for another 1-3 seconds and was done. The end piece was secured in place with individual welds 1-4 inches around the periphery.

A stitch weld can be very effective and efficient.

Reference: RWMA Manual 4th Edition

 

The answer to the question is maybe. It depends upon the coating and how the schedule and wheel geometry are set up to handle the change in the material surface. If this is a galvanized coating which will melt at a low temperature and could squeeze out of the weld joint area. Yes, one might expect a little more material pushed out of the seam area. This may or may not be noticeably more than with the bare material. This would need to be addressed in how the schedule and wheel are applied. However this could be a nonconductive paint or other coating which is another issue completely.

The answer is one would address the coated or non-coated material in a similar fashion as described in article:

HOW CAN YOU PREVENT SEAM WELDING BURRS WHEN WELDING 0.8 mm MATERIAL?

To address this question we will assume that we are making a liquid tight seam weld. In this process the welds overlap as the weld wheels roll forward. In a seam weld the process current is developed to allow for shunting current through the trailing welds which have already been made.

This part has tack/spot welds occasionally to hold the two surfaces in place. When the weld wheels approach one of these tack/spot welds the weld conditions change. As the wheel approaches, increasing shunting current can now travel forward through the parent material and the tack/spot weld. This forward shunting path is not part of the normal current allowance. This portion of the current is no longer passing through the intended seam weld. Our seam weld will be cooler because less current went through the desired seam weld. This might lead to the seam weld decreasing in size and possible failure to meet specification.

The best document for troubleshooting a seam welder would be:


AWS J1.2:2016 “GUIDE TO INSTALLATION AND MAINTENANCE of RESISTANCE WELDING MACHINES”

This document explains the importance of the various aspects of a proper installation. These installation guides give you many points to troubleshoot on an operating machine.

It also lists many undesirable weld results and what may be causing them.
It has a trouble shooting section. What to look for and where to go to correct the problem.

A seam welder is a resistance welder with circular rotating electrodes and flood cooling. All other factors are the same. The weld head is different in that it acts as a bearing and conductor. Wear of the bearings and sliding contacts is common over time. The conductive grease in the weld head normally needs to be replaced at the same time the weld head is overhauled.

Maintenance of the weld head should be left to seam welder manufacturers.

See: Article “CAN A WORN SEAM WELDING SHAFT BE REWORKED?”

I am going to assume that this question is being directed to the flood cooling water used in the seam welding process. This water may come from a cooling tower system and returns to the same system.  Normally it is treated in for particulates and the chemistry is evaluated and adjusted accordingly. This tower water will be reused over and over again.

More likely this flood water is on an independent system with a tap water make up which flows into a catch system to be recirculated on the seam welder itself for some period of time. The water itself is not harmful. It only is harmful from what it comes in contact with during seam welding. There is usually a considerable amount of steam, maybe smoke and some sparks possible. Some metallics can get into the water and sink to the bottom of the catch tank. The oils and other debris on the metal will change the water chemistry. The water will pick up what it is in contact with. It can be used over and over again for industrial cooling in the seam welding process. Make up water will be required to cover steam loss and evaporation.

In any long term reuse of this water in a plant water recirculating system or a dedicated seam welder recirculating system, the water should at the minimum be filtered for particulates and tested for pH, conductivity, and total dissolved solids.

Is used seam welding cooling water considered Potable water (Safe to Drink)?
NO!

Have a Question?

Do you have a question that is not covered in our knowledgebase? Do you have questions regarding the above article? Click here to ask the professor.